Accurate measurement of physical activity in children and adults is a challenging problem that is important to epidemiologists, exercise scientists, clinicians, and behavioral researchers. Although there are a number of indirect and direct methods to assess physical activity and energy expenditure, all current methods have serious, well-documented shortcomings vis-`a-vis application to free-living individuals. Recent research has shown that the most practical, objective method for measuring physical activity in free-living individuals is the use of portable activity monitors that are based on the joint monitoring of heart rate and accelerometry. Under R21 grant funding, the proposing team developed and evaluated a novel Physical Activity Monitor (the PAM-R21) for use in estimating both energy expenditure and the time spent at different activity intensity levels (e.g., sedentary/light, moderate, vigorous). Using heart rate and triaxial accelerometry, the PAM-R21, which uses artificial neural networks to convert heart rate and triaxial accelerometer data into estimates of energy expenditure, outperformed the only commercially-available integrated heart rate/acceleration activity monitor (viz., Actiheart) in terms of estimated energy expenditure, activity intensity level, and heart rate. The research proposed herein will leverage the previously-developed PAM-R21 to create and validate a low-profile, next-generation PAM-R01 monitor;validity and reliability testing will be performed in diverse populations that will include children, adults, and seniors. The PAM-R01 will also be compared with existing activity monitors in both structured and simulated free-living activities across a broad range of physical activity intensities, including the transitional periods between activities. The PAM-R01 will be small, lightweight, and unobtrusive, and will have wireless transmission capabilities;it represents an important step in the advancement of effective, accurate measurement of physical activity. PUBLIC HEALTH RELEVANCE: Physical activity is an important behavior that is related to reduce risk of a large number of negative health outcomes. However, the prevalence of physical inactivity remains unacceptably high;reducing the prevalence of inactivity is, therefore, a major focus of public health initiatives. The proposed development of the PAM- R01 activity monitor has the potential to remove one significant barrier that restricts research on the health effects of activity: the lack of an accurate, unobtrusive, and inexpensive device for measuring physical activity in free-living individuals. It also has strong potential for application in other areas, such as gait studies, pain management, and geriatric stability assessment and fall monitoring.